Problem: 10 pencils cost $18.10. Which equation would help determine the cost of 11 pencils?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 11 pencils. Since 11 pencils cost $x$ , we have the following proportion: $\dfrac{11}{x}$ We can write the fact that 10 pencils cost $18.10 as a proportion: $\dfrac{10}{\$18.10}$ The cost changes along with the number of pencils purchased, and so the two proportions are equivalent. $\dfrac{11}{x} = \dfrac{10}{\$18.10}$